Field of the Invention
The present invention relates to olivine lithium manganese iron phosphate cathode materials for lithium batteries and to methods for making such materials.
Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Not applicable
Lithium batteries are widely used as primary and secondary batteries for vehicles and many types of electronic equipment. These batteries often have high energy and power densities.
LiFePO4 is known as a low cost material that is thermally stable and has low toxicity. It can also demonstrate very high rate capability (high power density) when made with a small particle size and a good carbon coating. For these reasons, LiFePO4 has found use as a cathode material in lithium batteries. However, LiFePO4 has a relatively low working voltage (3.4V vs. Li+/Li) and because of this has a low energy density relative to oxide cathode materials. In principle, the working voltage and therefore the energy density can be increased by substituting manganese for some or all of the iron to produce a lithium manganese iron phosphate (LiaMnbFe(1-b)PO4 (LMFP)) cathode, without a significant sacrifice of power capability. However, structural stability and charge transport are adversely affected by replacing iron with manganese, and the specific capacities obtained have fallen significantly short of theoretical levels. Energy and power densities also are disappointingly low. In addition, battery cycling performance for LMFP electrodes often is less than desirable, due to a loss of capacity with cycling.
Attempts have been made to improve the performance of olivine lithium transition metal phosphate electrode materials by adjustments to the stoichiometry, i.e., the ratios of lithium, manganese, iron and the phosphate ions. Electrode materials containing an excess of lithium have been described, for example, in WO 2009/144600 and U.S. Pat. No. 7,842,420. Adding certain “dopants”, i.e., metals other than lithium, manganese and iron, also has been suggested in LMFP electrode materials having low manganese contents. See, e.g., WO 2011/025823, which mentions cobalt, nickel, niobium and vanadium as the “dopant” materials in an LMFP material in which the value of b is up to 0.6. See also U.S. Pat. No. 8,168,150, in which various other metals are suggested in an LMFP cathode material containing a large amount of iron.
There remains a desire to provide an olivine LMFP cathode material that has good specific capacity, high rate performance, and improved cycling performance.